This invention relates to a fin-tube heat exchanger for an air conditioner of the heat pump type which utilizes air as a heat source, and more particularly to a fin-tube heat exchanger suitable for use also as the outdoor coil of the air conditioner.
Conventional fin-tube heat exchangers will be described with reference to FIG. 1, FIGS. 2A and 2B and FIGS. 3A and 3B.
As shown in FIG. 1, generally, a fin-tube heat exchanger comprises a group la of fins arranged parallel to one another at equal intervals between any adjacent fins of which an air flow 4 passes; cylindrical fin collars 3 formed in the fin group 1a at a predetermined column pitch and a predetermined row pitch, and a group 2a of heat transfer tubes extending through the respective fin collars 3 and secured thereto in intimate contact therewith, through each of which heat transfer tubes a fluid flows.
Referring to a fin 1 of FIGS. 2A and 2B for the first conventional fin-tube heat exchanger, seat portions 5 are formed on the fin 1 in concentric relation to fin collars 3, respectively. A plurality of straight protuberances 6 of an angular cross-section are formed on the fin 1 between any two adjacent rows of the fin 1 collars 3, each having a ridge line extending in a direction in which each column of the fin collars extends. With this arrangement, when the air flow 4 passes through the group 1a of fins, the heat transfer is enhanced by a turbulence promoting effect.
Referring to a fin 1 of FIGS. 3A and 3B for the second conventional fin-tube heat exchanger, a plurality of juxtaposed, cut and raised portions 7 are formed in the fin 1 between any two adjacent fin collars 3 spaced from each other in the column direction. With this structure, when the air flow 4 passes through the group 1a of fins, the heat transfer is promoted by reducing the thickness of the boundary layer adjacent the front edge of the fin surface.
However, in the case where the fin-tube heat exchanger with the fins of FIGS. 3A and 3B is installed on the outdoor side of a heat-pump type air conditioner, frost forms on the front edges of the cut and raised portions 7, which have a good heat transfer performance, during a heating operation of the air conditioner when the outside air temperature falls, and soon the front edges are clogged with the frost. As a result, the heat transfer performance is abruptly lowered, thus posing a problem in that the heating operation could cease entirely due to this clogging.
The fin of FIGS. 2A and 2B is not provided with any cut and raised portion. Therefore, in the case where the fin-tube heat exchanger with these fins is installed on the outdoor side of the air conditioner, the heating operation can be continued for a longer time period even when the outside air temperature falls, as compared with the fin-tube heat exchanger with the fins of FIGS. 3A and 3B. However, since this heat exchanger is designed to promote the heat transfer by the turbulence promoting effect, its heat transfer performance is lower than that of the fin-tube heat exchanger with the fins of FIGS. 3A and 3B designed to promote the heat transfer by reducing the thickness of the boundary layer adjacent the front edge of the fin surface. Therefore, the fin-tube heat exchanger with the fins of FIGS. 2A and 2B has a problem in that it can not achieve a high performance and a compact construction of the heat-pump type air conditioner.